Since its discovery in the early 1980's HIV-1 pathogenesis has been of great importance for understanding progression of the disease. The entry phenotype of HIV-1 is determined by the HIV-1 Env protein. Env, the gp120/gp41 trimeric glycoprotein, studs the exterior of the viral particle, and is responsible for binding and initiatig entry into host cells. The HIV-1 envelope protein initially binds to cellular CD4, which triggers a conformational change to allow for the V3 loop region to bind a coreceptor, typically CCR5 or CXCR4. There is a great deal of confusion about the entry phenotypes HIV-1 can assume. Until recently HIV-1 tropism was believed to be determined primary by coreceptor binding, leading to two categories, CD4/CCR5 macrophage-tropic and CD4/CXCR4/T cell-tropic viruses. However, recent improvements in experimentation have revealed virus entry phenotype is not quite so terse. Only a fraction of CCR5-using viruses have the ability to infect macrophages, while the majority of all HIV-1 isolates are in reality R5 T-cell tropic, that is they can efficiently enter cells (with their high levels of CD4) and not macrophages (with their low levels of CD4). The tropism of HIV-1 is paramount to understanding HIV-1 pathogenesis and disease progression, directly affecting disease treatment and eradication. Current antiretroviral therapy (ART) reduces the level of viral RNA in plasma to below detection level using clinical assays (<50 copies/ml). Nevertheless, in approximately 75% of patients on suppressive therapy a persistent, stable low-level viremia (LLV) can be detected with laboratory assays. The sources of this LLV are currently unknown, but will be vital to being able to achieve a cure for HIV-1 infections. In this study, HIV-1 myeloid cell tropism will be studied in two distinct populations of patients; 1) patients who are late in disease progression, with CD4+ levels of <100 cell/m3, and who are viremic to determine if macrophage-tropic viruses ever reach a point of systemic infection, and 2) patients who are on successful ART, but still present with LLV, confounding eradication. HIV-1 present in plasma samples that meet the first requirements will come from two cohorts representing both subtype B and subtype C viral populations. These samples will be extracted, and individual env gene clones will be made using the single genome amplification process, followed by analysis for myeloid cell tropism (i.e. the ability to enter cells using low CD4) as defined by entry into Affinofile cells. Affinofile cells are a CD4 and CCR5-inducible cell line, which will allow for precise measurement of receptor and coreceptor usage for the determination of tropism. A similar approach with be used to determine the tropism of HIV-1 from donors on ART but who still maintain LLV using a larger scale approach for the initial RNA isolation steps. The results of this study will be invaluable for understanding the progression of HIV-1 pathogenesis given different environmental conditions, aiding in the design of therapies to progress the eradication of disease.